A: While adding underdrive pulleys is a common bolt-on upgrade for post-'79 Mustangs, we've learned in our tests that pulleys alone really don't yield any significant power gains on S197 ('05-'09) 'Stangs. We recommend installing pulleys as part of a package that includes a cold-air intake system, an after-cat or full exhaust, and a plug-in tune, similar to those we have featured in previous issues.
As a collective, the upgrades have shown us impressive rear-wheel power and torque gains on the chassis dyno, as well as the all-important, seat-of-the-pants oomph that enthusiasts yearn for when such changes are made.
You really shouldn't experience any problems by adding the underdrive pulleys. The swap is doable in the driveway provided you have the necessary tools (sockets/wrenches and a heavy-duty balancer puller) and an hour or so of free time to do the job.